FLOOD09674

• County: Statewide
• Community: Nationwide
• Basin: Statewide
• Title: Hydrology for Urban Land Planning - A Guidebook on the Hydrologic Effects of Urban Land Use
• Date: 1/1/1968
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

rises in flow, along with concomitant sediment
loads, control the stability of the stream chan-
nel. The two principal factors governing flow
regimen are the percentage of area made im-
pervious and the rate at which water is trans-
mitted across the land to stream channels. The
former is govel'lled by the type of land use; the
lattel' is governed by the density, size, and char-
acteristics of tributary channels and thus by the
provision of storm sewerage. Stream channels
form in response to the regimen of flow of the
stream. Changes in the regimen of flow,
whet he,' through land use or other changes,
cause adjustments in the stream channels to
accommodate the flows.

The volume of runoff is governed primarily
by infiltration characteristics and is related to
land slope and soil type as well as to the type
of vegetative cover. It is thus directly related
to the percentage of the area covered by roofs,
streets, .1nd other impervious surfaces at times
of hydrograph rise during storms.

A summary of some data on the percentage
of land rendered impervious by different de-
grees of urbanization is presented by Lull and
Sopper (1966). Antoine (1964) presents the
following dat" on the percentage of impervious
surface area in residential properties:

I.of Hi~r of
rp,~irln,tlfll GfeO
(WI}t)
6,000
6,000-15,000
15,000

Im!)f'r1;iO/lH
HI/TI(/N' (uta
(!)('(~(-'I't )

The percentage decreases markedly as size
of lot increases. Felton and Lull (1963) esti-
mate in the Philadelphia area that 32 percent
of the surface area is impervious on lots averag-
ing 0.2 acre in size, whereas only 8 percent
of the surface area is impervious on lots averag-
ing 1.8 acres.

As volume of runoff from a storm increases,
the size of flood peak also increases. Runoff
volume also affects low flows because in any
series of storms the larger thc percentage of (Ii-
rect runoff, the smaller the amount of water
available for soil moisture replenishment and
for ground-water stm'age. An increase in total
runoff from a given series of storms as a result
of imperviousness results in decreased ground-

80
40
25

.

water recharge and decreased low flows. Thus,
increased imperviousness has the effect of in-
creasing flood peaks during storm periods and
decreasing low flows between storms.

The pri!lcipal effect of land use on sediment
comes from the exposure of the soil to storm
runoff. This occurs mainly when bare ground
is exposed during construction. It is well
known that sediment production is sensitive to
land slope. Sediment yield from urban areas
tends to be larger than in unurbanized areas
even if there are only small and widely scat-
tered units of unprotected soil in the urban
area. In aggregate, these scattered bare areas
are sufficient to yield considerable sediment.

A major effect of urbanization is the intro-
duction of effluent from sewage disposal plants,
and often the introduction of raw sewage, into
channels. Raw sewage obviously degrades
water Wlality, but even treated effluent con-
tains dissolved minerals not extracted by sew-
age treatment. These minerals act as nutrients
and promot" algae and plankton growth in .
stream. This growth in turn alters the balanc ,
in the 'ltream biota.

Land "se in all forms affects water quality.
Agricultural use results in an increase of nu-
trients in stream water both from the excretion
products of farm animals and from commer-
cial fertilizers. A change from agricultural use
to residential use, as in urbanization, tends to
reduce these types of nutrients, but this tend-
ency is counteracted by the widely scattered
pollutants of the city, such as oil and gasoline
product." which are carried through the storm
sewers to the streams. The net result is gen-
erally an adverse effect on water quality. This
effect can be measured by the balance and
variety of organic life in the stream, by the
quantities of dissolved material, and by the
bacterial level. Unfortunately data describing
quality factors in streams from urban versus
unurbanized areas are particularly lacking.

Finally, the amenity value of the hydrologic
environment is especially affected by three
factors. The first factor is the stability of the
stream channel itself. A channel, which i.S
gradually enlarged owing to increased flood
caused by urbanization, tends to have unstabl
and unvegetated banks, scoured or muddy

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